This invention relates to a system for automatically controlling power tongs to obtain a predetermined limited torque while making up drill pipe.
In making up a pipe string for an oil well, a series of sections or joints having threaded ends are assembled in series as the pipe progresses into the well. The threaded connections are made by rotating a pipe joint by means of power tongs while holding the next lower joint stationary. It is generally desirable and frequently required that some automatic means be provided to apply a predetermined torque to the joints since overtorquing can damage the threads particularly with some modern pipe materials chosen for characteristics other than strength. On the other hand, undertorquing can result in leakage or unintentional disassembly of the joints. In some makeup operations, it is required not only that makeup torque be closely monitored but also that a permanent record be made of the makeup torque at each threaded connection.
The prior art includes numerous hydraulic power tongs, many of which are in the public domain, of the type having a hydraulic motor connected via a gear train to a rotary member having jaws for gripping the pipe. There are various known ways of limiting power tongs torque. One approach is simply to limit the hydraulic pressure differential across the motor; knowing the pressure-torque relationship of the motor and the ratio of the gear train, a predetermined makeup torque can thus be achieved. This approach is illustrated in U.S. Pat. No. 3,719,237, for example. Another known approach, used in the present invention, is to physically connect a torque or load transducer to the tongs. U.S. Pat. Nos. 4,199,032; 4,305,472; 3,745,820 and 4,091,451 illustrate tongs that are restrained against rotation with the pipe joint by a so-called snub line in which a load cell is mounted. The output of the load cell may be hydraulic pressure, as in U.S. Pat. Nos. 4,199,032 and 4,305,472; or an electrical output may be produced, as in U.S. Pat. Nos. 3,745,820 and 4,091,451. To achieve tongs cut-off at a particular torque level, the transducer may be linked to hydraulic control means in the tongs' hydraulic drive circuit. U.S. Pat. No. 4,305,472 shows one approach employing a shunt valve which, when opened, allows fluid to bypass the tongs. This approach is taken in the present invention.
Some of the prior art systems are indeed quite sophisticated, in the sense of being complex; applicant has found, however, that such systems have drawbacks so serious as to render them undesirable to potential customers. Besides the obvious expense of complex systems, they tend to be fragile, incapable of repair in the field by people not specially trained for the purpose, and may result in unacceptable delays when they fail. Owing to their cost, it is not feasible to maintain a suitable number of spares on hand, and where the driller must maintain a permanent record of makeup torque at each joint, the entire drilling operation may be brought to a halt by failure of the torque controller.
It is therefore an object of the invention to provide the industry with a power tongs control unit capable of reliably limiting applied tongs torque at a predetermined level, maintaining the variations from the desired torque from joint to joint to a very small figure, and making a graphic permanent record of makeup torque while avoiding the disadvantages of the prior art. To these ends the applicant has set out to create a system avoiding the complexities of prior systems, particularly avoiding complex computers, electronic modules and the like while still achieving the primary goal of reliable, accurate torque control.
To summarize the invention, a conventional power tongs is controlled by a system including a hydraulic snub line load transducer, the output of which communicates with a Bourdon tube load indicator or recorder, a limit switch positioned adjacent the Bourdon tube so as to be closed when a certain pressure is reached within the tube, and a solenoid actuated dump valve positioned between hydraulic lines to the tongs' hydraulic motor, the solenoid being electrically connected to a battery via the limit switch so that when the switch is closed by the Bourdon tube, the solenoid valve is opened, disabling the tongs. The limit switch is adjustably mounted relative to the Bourdon tube so that the threshhold pressure, and by inference a corresponding tongs torque, can readily be adjusted.
Another aspect of the invention is the provision of hydraulic means for maintaining substantial torque on the pipe after tongs operation has been halted. Such means comprises an adjustable choke in series with the shunt valve, between the tongs motor's hydraulic lines, whereby a substantial pressure differential is maintained across the tongs motor even after the shunt valve opens.
As a further feature of the invention, there is provided an adjustable electronic timer which, once the solenoid valve has been opened by the closure of the limit switch, continues to energize the solenoid, thereby holding the valve open for a predetermined time period. This timer prevents dangerous reapplication of torque to the tongs immediately following the initial disabling of the tongs, yet automatically resets the shunt valve in its closed position after enough time has passed for the operator to disable the tongs manually.
The present invention achieves all the stated objectives above while providing advantages of simplicity, low cost, repairability and durability over known more complex devices. These and other advantages of the invention will be apparent from a reading of the following description.